1. Field of the Invention
The present invention relates in general to a semiconductor process, and more particularly, to a method for fabricating a semiconductor device having a stacked-gate structure.
2. Description of the Related Art
A semiconductor memory device is typically divided into read/write memory and read only memory (ROM). The read/write memory is also divided into a dynamic random access memory (DRAM) and a static random access memory (SRAM). The DRAM is mainly used in memory applications, wherein a DRAM cell includes an access transistor and a storage capacitor.
Due to rapid development in memory research, the capacity of DRAM has quadrupled every three years. For example, 256 Mb DRAM has been developed, and also various studies on 1 Gb DRAM are in progress.
Currently, a doped polysilicon material is used to form the gate electrode of a semiconductor device, and a tungsten silicide layer is deposited on the doped polysilicon material in order to lower the resistance of the gate electrode. Additionally, in order to increase integration in DRAM, the line width of the gate electrode is reduced to reduce the overall size of a DRAM cell. However, the resistivity of the thin tungsten silicide layer is about 100 μΩ·cm, which is too high for use as the gate electrode material. Accordingly, further reduction in the resistance of the gate electrode is required to obtain a device capable of driving DRAM over 1 Gb with fine line width. There have been a number of studies on using tungsten as a gate electrode material, which has a resistivity of about 10 μΩ·cm. For example, a tungsten layer is deposited overlying a doped polysilicon layer and a tungsten nitride layer is formed therebetween to create a stacked-gate structure, where the tungsten nitride layer prevents increased resistance in the gate from reaction between the tungsten layer and the doped polysilicon layer. It is, however, difficult to control the resistance of the stacked-gate structure due to the poor thermal stability of the tungsten nitride layer. As a result, the reliability of devices is reduced.